In link aggregation, communication lines provided in each communication port and bundled together are handled as an integrated single communication line. As a standard specification, the link aggregation is specified in IEEE802.3ad (IEEE: The Institute of Electrical and Electronics Engineers, Inc.).
While the link aggregation has features such as a wider bandwidth of communication and redundancy, since a plurality of physical communication lines are integrated and managed, an operation and maintenance function (Operation Administration and Maintenance: OAM) is complicated. The OAM function in a network of an Ethernet (registered trademark, the same shall apply hereinafter) is specified in ITU-T Recommendation G.8013/Y.1731 (ITU-T: International Telecommunication Union Telecommunication Standardization Sector), as a standard specification.
Related techniques are disclosed in Japanese Laid-open Patent Publication No. 2009-17032, Japanese Laid-open Patent Publication No. 2010-154128, Japanese Laid-open Patent Publication No. 2007-243466, and the like.